A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever.
The present invention relates to pick and place machines. More particularly, the present invention relates to a method of calibrating pick and place machines.
Pick and place machines are used by the electronics assembly industry to mount individual components on printed circuit boards. These machines automate the tedious process of placing individual electrical components on the circuit board. In operation, pick and place machines generally pick up individual components from a component feeder or the like, and place the components in their respective positions on a circuit board.
During the placement operation, it is generally necessary for the pick and place machine to look at, or image, a given component prior to placement in order to adjust the orientation of the component for proper placement. Such imaging allows precise adjustments to be made to the component""s orientation, such that the component will be accurately placed in its desired position.
One standard type of pick and place machine uses a shadowing sensor, such as a LaserAlign(copyright) sensor available from CyberOptics(copyright) Corporation of Golden Valley, Minn. In a shadowing sensor, the object under test is rotated, and the effective width of the shadow (or an image of the shadow) is monitored on a detector. The dimensions of the object can be computed by monitoring the width of the shadow (or the shadow image). During start-up, the pick and place machine is calibrated so that any positional output from the sensor is mathematically related to the pick and place machine coordinate system. Once a correlation between the pick and place machine and the sensor output is known in X and Y, the pick and place machine can accurately place the object under test in its intended (X,Y) location on, say, a printed circuit board. There are also disclosed methods of calibrating the Z-height of a nozzle, so that the pick and place machine can repeatably place the object onto the intended place at the correct Z height. However, the methods disclosed for calibrating pick and place machines in (X,Y) and in Z are specific to the type of sensor in the pick and place machine.
Another type of pick and place machine uses an on-head linescan sensor to image the component while the placement head is traveling. An on-head sensor, as used herein, refers to a sensor which travels with the placement head in at least one dimension, so as to sense the orientation of the component while the component travels to the circuit board. This is in contrast to off-head systems, where the component is transported to a stationary station to sense the orientation of the component, and from there, the component is transported to the circuit board. A linescan sensor, as used herein, is an optical sensor comprised of a plurality of light sensitive elements that are arranged in a line such that the sensor acquires a single line of the image in a given time period. By translating the linescan sensor relative to the entire component and storing a plurality of the acquired lines, the component image is realized and X, Y and xcex8 orientation is then calculated using this scanned image.
Placement machines incorporating on-head linescan sensing technology are very flexible in the types of components that they can place. The on-head linescan sensor is able to directly image components such as chip capacitors, Quad Flat Packs (QFP), TSOP, Ball Grid Arrays (BGA), CSP, and flip-chips. The video output of the linescan camera allows a video processor to compute the orientation of the component. Based on knowledge of the desired orientation of the component and the present orientation, the pick and place machine corrects the orientation of the component and places it on a printed circuit board. The linescan image can also provide inspection information about the component to be placed. Also, placement machines incorporating on-head linescan sensing are very fast compared to off-head sensing technologies since the step of visiting a fixed inspection station to measure pick-up offset errors is eliminated. To increase the accuracy of pick and place machines using on-head linescan sensing technology, however, careful calibration of the linescan sensor and its physical relationship to other parts of the placement machine should be performed.
A method of calibrating a pick and place machine having an on-head linescan sensor is disclosed. The calibration includes obtaining z-axis height information of one or more nozzle tips via focus metric methods, including a Fourier transform method and a normalized correlation method. Additionally, other physical characteristics such as linear detector tilt, horizontal scale factor, and vertical scale factor are measured and compensated for in the process of placing the component. Nozzle runout, another physical characteristic, is also measured by a sinusoidal curve fit method, and the resulting Z-height calibration data is used to later place the component.